Pressure swing adsorption (PSA) is a process that may be used to remove impurities from an impure hydrogen gas stream by selective adsorption of one or more of the impurities present in the impure hydrogen stream. The adsorbed impurities may be subsequently desorbed and removed from the PSA assembly. PSA is a pressure-driven separation process that utilizes a plurality of adsorbent beds. The beds are cycled through a series of steps, such as pressurization, separation (adsorption), equalization, depressurization (desorption), and purge steps, which also may be referred to as a PSA cycle, to selectively remove impurities from the hydrogen gas and then desorb the impurities. The operation of PSA assemblies may include a plurality of operating states, which also may be referred to as PSA states and/or PSA operating states. These states may include, but are not limited to: a hydrogen-purifying state (i.e., a running state), a shutdown state, a dormant state, and a startup state.
The dormant state refers to when the PSA assembly has completed its shutdown state and is not being used. Typically, the PSA assembly will be transitioned from its running state to its shutdown state and then to its dormant state. In some PSA assemblies, the adsorbent beds may be depressurized during the shutdown state, and thereby will not be pressurized in the dormant state, whereas in other PSA assemblies, the adsorbent beds may be pressurized in these states.
The startup state may include a series of steps selected and/or configured to transition the PSA assembly from the dormant state to the hydrogen-purifying state. When a portion (or all) of the PSA assembly (such as one or more adsorbent beds) is not pressurized when in the dormant state, the assembly may need to be pressurized, such as during the startup state, prior to initiation of other PSA process steps in order to provide the pressures and/or pressure differentials necessary for operation of the PSA assembly.
Conventionally, this pressurization may include pressurizing the PSA assembly with a mixed gas stream, such as a reformate stream produced by a hydrogen-producing fuel processor, a hydrogen-generation assembly, or another source of impure hydrogen gas. However, this mixed gas stream may contain a significant concentration of impurities, namely, a concentration of impurities that exceeds an acceptable level of impurities during operation of the PSA assembly in the hydrogen-purifying state. Pressurization of the PSA assembly with the mixed gas stream during startup of the PSA assembly may result in flow of the mixed gas stream or other impure gas stream into regions of the PSA assembly that are intended to contain primarily purified hydrogen gas during operation of the PSA assembly in the hydrogen-purifying state. This may result in reduced hydrogen purity during initial operation of the PSA assembly in the hydrogen-purifying state, the need to utilize hydrogen gas as a purge stream during the startup state to remove these impurities, and/or longer and/or more complicated startup procedures.